### Origin of Earth's Water
How Earth acquired its water is a subject of ongoing debate among scientists. Three viewpoints propose different mechanisms for the origin of Earth’s oceans.
Viewpoint 1
Earth formed in the hot inner region of the protoplanetary disk, where temperatures were too high for water ice to condense. Consequently, the proto-Earth was initially completely dry. Earth's water was delivered much later, approximately years ago, during the Late Heavy Bombardment. This water was brought by carbonaceous chondrite meteorites from the outer asteroid belt that migrated inward and collided with Earth.
Viewpoint 2
Earth formed from dust grains that already contained chemically bound water. During accretion, water-bearing silicate minerals in the warm inner solar system clumped together to form the planet. Rather than being dry, the early Earth trapped water deep within its mantle. Over millions of years, this water was released to the surface as water vapor through volcanic outgassing, eventually condensing to form the oceans.
Viewpoint 3
While Earth was initially dry, meteorites from the asteroid belt contain too little water to account for the current volume of the oceans. Instead, Earth's water was delivered after the planet's crust solidified by icy comets originating from the cold outer regions of the solar system beyond Neptune. These comets deposited water directly onto the surface during impact.
Based on Viewpoint 2, which of the following statements best describes how Earth acquired its water?
- Water was incorporated into Earth during its formation from local silicate dust grains and was later released from the interior.Answer
- BWater was delivered to Earth after its formation by carbonaceous chondrite meteorites originating from the outer asteroid belt.
- CWater was deposited directly onto Earth's surface after its crust solidified by icy comets from the outer solar system.
- DWater was brought to Earth during its accretion by icy comets that migrated from beyond Neptune.